Orthogonal Frequency Division and Multiplexing (OFDM) systems are being applied in modern digital technology for data transmission. In this principle, before being transmitted a digital data stream is converted by mapping into complex-value symbols and split up into a multiplicity of partial signals, each of which is transmitted separately on an individual carrier. In the DVB-T (Digital Video Broadcasting) system 1,705 or 6,817 such individual carriers are used, for example. In the receiver these items of partial information are recombined into a whole item of information of the sender-side digital data stream. This OFDM system is known and is described in greater detail in, for example, HERMANN ROHLING, THOMAS MAY, KARSTEN BRUNINGHAUS and RAINER GRUNHEID, Broad-Band OFDM Radio Transmission for Multimedia Applications, Proceedings of the IEEE, Vol. 87, No. 10, October 1999, pp. 1778 ff.
In modern transmission technology via radio channels a frequency hop method is often applied to compensate fading intrusions in the channel, in which the useful information is transmitted in sending frequency intervals (hops), changing in hops, of, for example, only 5 ms duration.
If these two systems known per se are applied in combination and OFDM signals are transmitted via radio channels according to the frequency hop method, it must be ensured that the receiver is synchronized exactly in respect of frequency and time to the OFDM blocks transmitted in the individual hops. Owing to movement by sender and/or receiver or owing to differences in the frequency of sender and receiver, Doppler and frequency shifts of the individual carriers of the OFDM can arise, which, because of the frequency agility, are not constant, but change on each of the successive frequency intervals, corresponding to the speed of movement or the transmission frequency on the respective frequency interval. It is therefore necessary for the receiver to be synchronized exactly to the individual carriers of the OFDM signal in respect of frequency in each of the individual successive transmitting frequency intervals.
Moreover, care must be taken that the receiver is also synchronized in respect of time exactly to the start of the OFDM signal packets (OFDM blocks). Owing to differences in running time, for example as a function of the distance between sender and receiver, these OFDM blocks do not always arrive at the receiver at the same desired time.